In general, the state of the gas depends on both the temperature, via the
recombination rates and collisional ionization rates, and on the radiation
field, via the photoionization rates. This combined dependence makes a
display of the ionization balance cumbersome in the absence of some other
simplifying assumption. Figure 5 shows the ionization balance in the coronal case,
i.e. when the radiation field is negligible, as a function of temperature. This can
be compared with other similar calculations such as, e.g. [Arnaud and Rothenflug 1985].